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Explain how to flatten a multi-level nested JSON file while loading it into BigQuery.

SQLeasy0.4 min readPremium

**Architectural Logic**: UNNEST for arrays; dot notation for structs; nested UNNEST for multi-level. **Approach**: `SELECT id, item.name, item.price FROM t, UNNEST(t.items) AS item`. Multi-level: nested UNNESTs or JSON functions (JSON_VALUE, JSON_QUERY). **Alternative**: Load as...

🤖 Analyze Your Answer
Frequency
Low
Asked at 1 company
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
Aarete
Key Concepts Tested
bigqueryetl

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This easy-level SQL question appears frequently in data engineering interviews at companies like Aarete. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (bigquery, etl) will help you answer variations of this question confidently.

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Start by clearly defining the core concept being asked about. Interviewers want to see that you understand the fundamentals before diving into implementation details. Structure your answer with a definition, then explain the practical application with a concise example.

Expert Answer
81 words

Architectural Logic: UNNEST for arrays; dot notation for structs; nested UNNEST for multi-level. Approach: SELECT id, item.name, item.price FROM t, UNNEST(t.items) AS item. Multi-level: nested UNNESTs or JSON functions (JSON_VALUE, JSON_QUERY). Alternative: Load as JSON column; flatten in dbt or ETL. Scalability: Complex nesting may need recursive or staged processing. Cost: Flatten in load vs in query—load-time can reduce repeated cost. Best Practice: Understand schema; use UNNEST for arrays; preserve structure if needed; validate null handling. Preprocess for very complex structures.

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According to DataEngPrep.tech, this is one of the most frequently asked SQL interview questions, reported at 1 company. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.

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